Composition and method for forming black coatings for metals



United States Patent COB POSITION AND METHOD FOR FORMING BLACK COATINGS FOR METALS Peter Borghetti, Chicago, Ill., and Walter R. Cavanagh,

Detroit, Mich, assignors to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application March 27, 1953, 'Serial No. 345,238

6 Claims. (Cl. 1486.17)

The present invention relates to black coatings for metallic surfaces and to a method for making the same. More particularly, the invention concerns coatings for a variety of metallic substrates which provide both increased resistance to corrosion and a permanent black color.

Coatings capable of resisting corrosion and a variety of processes for producing such coatings are known. Phosphate coatings are used extensively for this purpose. Corrosion resistant phosphate coatings vary in weight and grain size depending upon one or more of a number of factors including the oxidizing agent employed, ,the preliminary preparation of the metal surface, the temperature and time of operation, etc., but such coatings are usually characterized by a gray to dark gray ap earance. Such coatings are not sufficiently attractive --,to be considered a finish coating, and an overlayer of paint,

.lacquer, varnish, etc. is applied where appearance is important. Black oxide finishes have been heretofore produced from alkaline solutions, but so far as is known, there have been .no black phosphate coatings.

The principal object of this invention is to provide a black corrosion resistant coating containing phosphate on metals such as iron, steel, nickel and alloys of iron and nickel.

Another object is to provide a method for simply and inexpensively producing a black corrosion resistant coating on metallic-surfaces.

.Other objects and advantageous features of thejnven- .tion will become apparent upon considering the detailed description in its entirety.

In accordance with the present invention it has been found that a corrosion-resistant coating having a black color may be formed on a metallicsurface fromana'queous acidic solution which contains as the essential coating producingingredients the phosphate ion, 'the sulfide .ion,

at least one .metallic ion from the group consistingof zinc, manganese and cadmium, and at leastonemetallic ion whose sulfide is black. In order for the coatings whichare obtained from such solutions to be adherent,

permanently black and useful as a final finish,rit'has been found necessary to carefully control therelativeproporti-ons .of the several ingredients as will be set forth in greater. detail hereinafter. The metallic surfaces on which the coatings of thisinvention may be successfully formed are'the ferrous metals including iron and steel, nickel and the alloys of iron and nickel.

The present invention is based on thediscovery that .a

phosphate coating for a metallic surface, as above dofined, can be renderedblack as it is formed on themetal surface by incorporating into an aqueousacidic phosphate solution of otherwise conventional composition, an ,extremely minor proportion of free sulfide ion and a small amount of a metallic ion which will form with the free sulfide a black sulfide as a coating is formed on the metal. JItLis believed, althoughit is not desired to be bound by the ostulated reason forthe formation of'the black coatings which are obtained, that the final coating 'ice .On the metal surface contains a predominant proportion of zinc, iron, manganese or cadmium phosphate and a minor proportion of nickel or other metal sulfide distributed uniformly throughout the phosphate coating. The metallic sulfide, although constituting an extremely minor proportion of the total coating, is thought to alter the normal gray appearance of the coating to a black appearance which is pleasing to the eye and suitable as a finish coating, when oiled.

The phosphate ion, which may be present in the aqueous acidic solution as free phosphoric acid or a phosphate of Zinc, iron, manganese or cadmium performs its conventional function, now Well known in the art, and contributes heavily to the corrosion resistance of the coating which is formed. Numerous attempts have been made to eliminate phosphates from the solutions, and relatively dark deposits have been obtained in its absence, but such deposits were either subject to the deficiency that they had relatively low corrosion resistance and/or the dark color was only temporary and soon faded away.

The phosphate ion may be conveniently introduced as a metallic phosphate such as zinc phosphate, manganese .phosphate or cadmium phosphate, or partly as a metallic phosphate and partly as phosphoric acid or entirely .as phosphoric acid. In the event, however, that the phosphate ion is supplied as phosphoric acid, sufficient Zinc, manganese or cadmium should be introduced to form metallic phosphate with the major portion of .the phosphoric acid. Of the metallic phosphates including Zinc, manganese and cadium phosphates which have been found to be suitable, zinc phosphate is preferred. The phosphate ion may be present in a concentration varying between about 5 and grams/liter, but no particular FORMULA I P04 ion 5- .100 grams/liter. Sulfide ion 0.0060.12gram/liter. Zinc ion '1.0 (minimum)gram/liter. Nickelion 0.4 (minimum) gram/liter. Ferrous ion 1.25 (minimumlgrams/liter.

The sum of zinc+nickel+ferrous ions equal toat least 5 grams/liter.

In:the compositions of Formula I, the sum of the zinc, nickel and ferrous ion content may exceed the minimum of 5 grams per liter by a large amount and in the usual commercial installation is normally in the range of -20 to 30 grams/liter. Proportions of metallic ions as great as about 50 grams/liter have been used without experiencing detriment to the depth of the black color or the corrosion resistance of the coatings. The quantities given in Formula I for the metallic ions are the minimums below which the benefits of the invention may not be realized. Such solutions are preferably operated at an acidity of 25-35 points, but may vary in acidity over a relatively wide range similar to that which is conventionally employed for forming phosphate coatings which are not black.

The zinc ion may be substituted for by the manganese or cadmium ion in equivalent proportions, but when manganese or cadmium are present as a substitute for zinc,

it is necessary to increase the proportions of nickel into the solution in the form Sulfur containing compound.

M dihydrogen phosphate and where manganese or cadmium are used the same procedure may be followed.

The nickel ion of Formula I may be substituted for by any metal ion, the sulfide of which is black, and which will not oxidize the ferrous ion in the aqueous acidic solutions of the invention. Included in this category are nickel, copper, lead, tin, cobalt, titanium, antimony, silver, thorium, iridium, molybdenum, platinum, tantalum, tungsten, vanadium, uranium and mercury. The only metallic ion tested whose sulfide is black which did not produce a black coating in solutions employing quantities set forth in Formula I is thallium, and it is believed that the failure to obtain a black coating is due to the ability of thallium to oxidize the ferrous ion to the ferric ion. Nickel ion is preferred because of the somewhat superior depth of blackness and the uniformity of results obtained with its use. These metallic ions may be introduced conveniently into the solutions of the invention as a metallic salt such as a nitrate chloride or sulfate. The minimum proportion of nickel ion which is required is 0.4 gram/ liter or 0.04% by weight, and where another of the metallic ions above suggested as usable are employed, a proportion of that metallic ion equivalent to the minimum for the nickel ion or a greater quantity should be used. As stated above, the usual solution contains proportions of each of the metallic ions well above the minimums set forth inasmuch as erratic operation and insufficiently black coatings are obtained when the concentration of the metallic ions drop below the specified minimum quantities. Within the class consisting of metals whose sulfides are black in color,

there is a smaller group of metals which, because of their availability, cheapness and tion of uniformly deep the superiority in the formablack coatings relative to the other ,metals in the broad class, constitute a preferred group for the purposes of this invention. This group consists of .nickel, lead, tin, cobalt, copper and titanium.

The ferrous ion may be introduced into the solution as a ferrous metal salt or may be built up in the solution, where the metal being treated is a ferrous base material, by attack on the substrate metal. In the usual case it is more desirable to add the requisite quantity of a ferrous metal salt. Of the total metallic ion content of the solutions of Formula I, at least 25% should be the ferrous ion and as much as 80% of the total may be the ferrous ion so long as the expressed minimum quantities of the zinc and nickel ions, or their equivalent, are concurrently present.

As may be noted from Formula I, the proportion of sulfide ion which may be used is extremely small and rather limited in allowable variation of concentration. When the proportion of sulfide ion drops below 0.006 gram per liter, the coating which is obtained fromthe solutions of Formula I is not black, and when the proportion of sulfide ion exceeds 0.12 gram/ liter, a large amount of precipitate is formed in the solution and a non-uniform dark gray coating is obtained. The sulfide ion may be introduced of any sulfur compound which will release sulfur ions having a valance of -2 in the acid medium of Formula I. In addition to the readily available alkali metal sulfides and alkali metal oxygen containing sulfur compounds, hydrogen sulfide may be used. For practical reasons, it is preferred to employ the alkali metal-sulfur compounds such as sodium sulfite, sodium thiosulfate, sodium sulfide, potassium dithionates and tetrathionates, alkali metal hyposulfites and polysulfides.

The sulfide ion is thought to be released in the acid medium of Formula I relatively slowly and in order to establish and maintain the of active sulfide ion in the solution, it is best to provide in the solution an excess of the sulfide ion within the A suitable concentration of sulfur containing compound is 1.5 to 2.0 grams per liter of sodium thiosulfate, Na2S2O3.5H2O or its equimolecular equivalent, with respect to sulfur, in other sulfur containing compound.

proper proportions ing, with the depth of the greater on the links dipped in oil.

4 Improved adherence, uniformity of quality and corrosion resistance are obtained from the use of solutions hav ing the composition set forth in Formula II.

Ferrous ion Minimum 1.25 grams/ liter. Nitrate ion 2.5-30 grams/liter.

The nitrate ion which in the solutions of this invention acts as an oxidizing agent to decrease the time required to form a coating may be replaced by any of the other oxidixing agents which do not oxidize the ferrous ion to the ferric ion and which are well known to be suitable in phosphate solutions of the coating phosphate type, that is, phosphates of zinc, manganese, and cadmium.

The method of this invention comprises the steps of contacting the metal surface to be treated with a-solution having a composition falling within the ranges of proportions set forth in Formulas I or II. The metal may be contacted by immersion in the presence of conventional oxidizing agents, and for best results the solution should be at a temperature between about F. to 210 F. Immersion for about 15 to 30 minutes in the solution is sufficient to produce a black adherent coating. After removal, the coated article may be water rinsed and is preferably immersed in a dilute aqueous solution of chromic acid. A chromic acid solution containing from about 1 to 20 grams per gallon of CrOz is satisfactory.

After rinsing in chromic acid the coated surface is preferably oiled with any of the oils conventionally employed in treating chemically formed coatings in the conventional manner.

The examples given below illustrate in greater detail the compositions and method of the invention.

Example 1 An aqueous solution was built up by admixing the following materials in sufficient water to make 4 liters:

The temperature of the aqueous admixture was raised gradually'to about F. with periodic stirring and maintained for about 1% hours. Two grams of sodium 'thiosulfate was then added to the hot bath and stirred for about 15 minutes to obtain complete dissolution. of the thiosulfate and equilibrium conditions in the bath. An analysis showed the bath contained 0.27% ferrous iron.

A number of grit blasted fifty caliber mild steel cartridge links were immersed in the bath and maintained therein for 20 minutes. The coated links were then removed from the bath, rinsed in water, and dried. Some of the links were then dipped in Parcolac 2B, an oil containing stearates, an organic solvent and a rust inhibitor, withdrawn and allowed to drain. Other links were dipped in a dilute aqueous solution of chromic acid (2 to 20 grams/ gallon) prior to dipping in oil.

All of the links were coated with a black adherent coatblackness being somewhat Example 2 Grams/liter Grams Zinc dihydrogen phosphate, Zn(H2PO4)z.2H2O 3.2 Mercurous chloride, HgCl 0.826 Phosphoric acid (75%), H3PO4 4.1 Stannous chloride, SnClzlHzO 1.345 Ferrous chloride, FeClz 3.5 Silver nitrate, AgNOa 0.585 Zinc chloride, ZnClz 4.8 5 Mercuric chloride, HgClz 0.941 Nickel sulfate, NiSO4.6H2O 0,9 Cobaltous chloride, C0Cl2.6H2O 0.837 Sodium thiosulfate, 'NazSzOa 0.5 Throium nitrate, Th(NO3)4.4H2O 1.919 Analysis showed the ferrous iron content to be 0.46%, Stannic chloride, SnClaSHzO 1.221 Titanium tetra chloride, TiCl4 0.642 Cartr dge links processed through the above solution Antimony chloride, Sbcl3 7 2 had a similar appearance to those of Example 1. ceppcr chloride, CuClzZHZO 0505 Example 3 Lead chloride, PbClz 0.945 Mild steel cartridge links processed in each of the above The proc'?dure of Example 111 h makeup baths at 195 F. to 200 F. for to minutes and and processing wasfollowed With-a 4l1ter solution of the 15 Subsequently processed in accordance with the method of followmg Composltlon: Example 1 were all provided with an adherent, black Grams/liter corrosion resistant coating. Zinc dihydrogen phosphate, ZI1(H2PO4)2.2H2O 3.2 Thallium chloride, TlCl3.4H2O, Was added to a 500 ml. Phosphoric acid (75 'H3PO4 4.1 portion of the basic solution and no black coating was Ferrous chloride, FeClz 2.5 20 obtained under the same processing conditions. No black Zinc chloride, ZnClz 2.3 coating was obtained even after the proportion of thallium Nickel sulfate, NiSO4.6HzO 0.9 chloride was doubled. Sodium thiosulfate, NazSzOs 0.5 Analysis showed the ferrous iron content to be 0.33%. Example 6 Cartridge links processed through the above solution To Separate Portions of the basic/(bath of E p e 1, h d a i il appearance to those of E l 1 and 2 small quantities of an aqueous solution of sodlum sulfide Representative links f Examples 1 2 and 3 were having a concentration of 29.4 grams/liter sodium sulchecked for corrosion resistance in the conventional acfide Were added and mlld steel links Processed celerated salt spray test and all were found to have good Gordan? Wnh Procedur? Example sulfifie corrosion resistance which for comparative purposes was Fentranons below 7 faded to Produce black coat approximately the same as that obtained with conven- Whereas c?n centrauons of sulfide 0-012% tional phosphate coatings. The best corrosion resistance P a Rreclpltate and Poor black coatmgs- Was obtained with the links which were dipped in the What clalmfefi 1S5 dilute aqueous chromic acid rinse prior to oiling. A compos}tlon produclhg f coatmgs on P faces of steel, 1ron, nickel and iron-nickel alloys which Example 4 comprises an aqueous acidic solution comprising as the essential coating producing ingredients 5 to 100 grams/ A cPmmerclal 250 Zinc ph9s phate bath upon liter of the phosphate ion, at least about 0.125% of the analysls Showed the followlng composmoni LO ferrous ion, at least about 0.1% of a first metallic ion Zinc "grams/liter" 30 selected from the group consisting of zinc, manganese Ferrous ion 25 and cadlurn ions, said first 1011 being present in an amount Nitrate ion do 10 suflicient to form metallic phosphate with the ma or porphosphate ion do 15 11011 of saidphosphate ion, about 0.0006 to 0.012% of Free acid 5'5 tne sulfide ion and a second metalllc ion distinct from Total aCI-d do 3O first metallic ion which forms a black sulfide and which This bath was then modified by adding thereto: will not oxidize the ferrous 10m in sa1d aqueous acidic Pounds solution, the total metallic ion content being greater than Sodium thiosulfate, NazSzOs 3%. about Nickel Sulfate Nismflho 6% A composgtwn r p s blaqk coatmgs on u faces of steel, iron, nickel and iron-nickel alloys WhlCh After modification, an analysis showed that the bath concomprises an aqueous acidic solution comprising as the r i d 075% i k l i d 0,15% to (120% diu essential coating producing ingredients about 1% to about thi lf t 5% P04 ion, at least about 0.1 of a first metallic ion se- Over 13,000 square feet of mild steel stock was procleeted from the g p ehhsisiihg of Zinc, manganese and essed h h h b h at 195 F 2()() F on a 20 cadmium, said first ion being present in an amount sufminute immersion cycle. The stock was removed from fieieht t0 form metallic Phosphate with the major P the bath, Water rinsed, chromic acid rinsed and oiled as tieh Of Said Phosphate 1011, the ferrous at least about described in Example 1. Good adherent black coatings 004% of eh h metallic Selected from the were obtained having good corrosion resistance. EF P Conslstlhg 0f hlckel, e tlh, Cobalt, copper e titanlum, the total metallic ion content in said solution Example 5 being greater than 0.5%, the ferrous ion constituting A b h h t b th d h the from 25% to about 80% of said total metallic ion coni 05p a e a was prepare avmg tent, and about 0.0006% to about 0.012% of the sultollowmg analysis: fida ion. Zinc ion "grams/liter" 3. A composition for producing a black coating on sur- Nitrate ion 10 faces of steel, iron, nickel and iron-nickel alloys Which Ferrous ion 25 comprises an aqueous acidic solution containing as the Phosphate ion 15 essential coating producing ingredients 5100 grams/ Free acid "points" is liter of the phosphate ion, at least about 0.125% of the Total acid 315 ferrous ion, at least about 0.1% of the zinc ion, said zinc ion being present in an amount sufficient to form zinc To separate 500 ml. portions of this basic solution, dihydrogen phosphate with the major portion of said varying quantities of metallic salts, other than nickel phosphate ion at least about 0.04% of the nickel ion, salts, were added together with sutficient sodium thiosuland the sulfide ion, the sum of the quantities of the metalfate to obtain a cencentration of 1.5 grams/liter NazSzOs. lic ions being at least 0.5% by weight and at least 25% of said total being ferrous ion, the sulfide ion being present in. an amount between about 0.0006% and about 0.012%. "4'."A"c'omposition for producing a black corrosion resistant'coating on the surface of steel, iron, nickel and alloys of iron and nickel which comprises an aqueous acidic solution consisting essentially of at least about 1.0% P04, 2.5 to30 grams/liter of the nitrate ion, at least about .04% nickel ion, at least about 0.1% zinc ion, said'zinc ion being present in an amount sufficient to form zinc dihydrogen phosphate with the major portion of" said phosphate ion and theferrous ion, the sum of the nickel, zinc and ferrous ions being at least 0.5%, the ferrous ion constituting between 25% and 80% of the total metallic ion content, and the sulfide ion in an amount between '.0006% and about 0.012%.

' 5. A method for producing a black corrosion resistant coating on the surfaces of steel, iron, nickel and ironnickel alloys which comprises the steps of contacting the metal surface with an aqueous acidic solution comprising as the essential coating producing ingredients 5 to 100 grams/ liter of the phosphate ion, at least about 0.125% of the ferrous ion, at least about 0.1% of the zinc ion, said zinc ion being present in an amount sufficient to form zinc dihydrogen phosphate with the major portion of said phosphate ion about 0.0006% to 0.012% of the sulfide ion and a metallic ion other than zinc which forms a black sulfide and which will not oxidize the ferrous ion in said aqueous acidic solution, the total metallic ion content being greater than about 0.5%, maintaining said solution in contact with said surface until a coating is formed thereon and thereafter oiling the coating.

6. A method for producing a black corrosion resistant coating on the surfaces of steel, iron, nickel and ironnickel alloys which comprises the steps of contacting the metal surface with an aqueous acidic solution comprising as the essential coating producing ingredients about 1% to about 5% PCs ion, at least about 0.1% of the zinc ion, said zinc ion being present in an amount sufficient to form zinc dihydrogen phosphate with a major portion of said phosphate ion, the ferrous ion, at least about 0.04% of at least one metallic ion selected from the group consisting of nickel, lead, tin, cobalt, copper and titanium, the total metallic ion content in said solution being greater than 0.5%, the ferrous ion constituting from 25% to about of said total metallic ion content, and about 0.0006% to about 0.012% of the sulfide ion, maintaining said solution in contact with said surface until a coating is formed thereon and thereafter oiling the coat- References Cited in the file of this patent UNITED STATES PATENTS 1,317,351 Chadwick Sept. 30, 1919 2,121,520 Curtin June 21, 1938 2,302,643 Thompson Nov. 17, 1943 2,554,139 Drysdale May 22, 1951 FOREIGN PATENTS 8,667 Great Britain Jan. 24, 1907 of 1906 447,176 Great Britain May 13, 1936 588,241 Great Britain May 19, 1947 

1. A COMPOSITION FOR PRODUCING BLACK COATINGS ON SURFACES OF STEEL, IRON, NICKEL AND IRON-NICKEL ALLOYS WHICH COMPRISES AN AQUEOUS ACIDIC SOLUTION COMPRISING AS THE ESSENTIAL COATING PRODUCING INGREDIENTS 5 TO 100 GRAMS/ LITER OF THE PHOSPHATE ION, AT LEAST ABOUT 0.125% OF THE FERROUS ION, AT LEAST ABOUT 0.1% OF A FIRST METALLIC ION SELECTED FROM THE GROUP CONSISTING OF ZINC, MANGANESE AND CADIUM IONS, SAID FIRST ION BEING PRESENT IN AN AMOUNT SUFFICIENT TO FORM METTALIC PHOSPHATE WITH THE MAJOR PORTION OF SAID PHOSPHATE ION, ABOUT 0.0006% TO 0.012% OF THE SULFIDE ION AND A SECOND METTALIC ION DISTINCT FROM FIRST METALLIC ION WHICH FORMS A BLACK SUFLFIDE AND WHICH WILL NOT OXIDIZE THE FERROUS ION IN SAID AQUEOUS ACIDIC SOLUTION, THE TOTAL METALLIC ION CONTENT BEING GREATER THAN ABOUT 0.5%. 